Underfilling process in a molded matrix array package using flow front modifying solder resist

ABSTRACT

Placing a flow modifier on a package substrate to create two flow fronts on a molded matrix array package. A flow modifier may be laid on a package substrate to a height that blocks off the bottom of other substrates (e.g., dice) coupled to the package substrate. By separating the top flow front and the bottom flow front, this process prevents the top flow front from wrapping around the sides of the substrates and trapping air below each substrate and in front of the bottom flow front.

This is a Divisional Application of Ser. No. 10/287,318 filed Nov. 4,2002, which is presently pending.

BACKGROUND

1. Technical Field

An embodiment of the invention relates generally to manufacturingelectronic circuit assemblies, and in particular to the molding processof a molded matrix array package.

2. Description of the Related Art

Molded matrix array packages are underfilled and overmolded in a singlestep. This may include several flip chips mounted on a substrate. Afterunderfilling and overmolding all of the flip chips on the substrate, theindividual flip chips may be singulated into packages. Variations in themold flow both on top and on bottom of the flip chips can cause multipleproblems in the molding process. In addition, because the moldingcompound can flow around and over each die prior to a molding compoundunderfilling the die, air may be trapped under the die. The trapped airunder the die may cause a void that will decrease reliability in thepackage. To reduce the size of the void in a conventional process, a lowpressure region may be created in the mold chase to eliminate trappedair. In addition, a small hole may be poked through the substrate undereach die to allow air in the void to escape as the molding processfinishes. However, using a low pressure region or poking a hole in thesubstrate may increase the cost of producing the package and may reducethe reliability of the package.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by referring to the followingdescription and accompanying drawings that are used to illustrate theembodiments of the invention. In the drawings:

FIG. 1 shows a side view of an embodiment of the invention showing across section of flow front modifiers.

FIG. 2 shows a top view of an embodiment of the invention indicatingdevice placement locations.

FIG. 3 shows a side view of an embodiment of the invention showing across section at a right angle to the view of FIG. 1, indicating flowmodifier height indicators.

FIG. 4A shows a top view of an embodiment the invention after the flowmodifier has been applied, but before the devices have been coupled tothe package substrate.

FIG. 4B shows a side view of the embodiment of FIG. 1 after the flowmodifier has been applied, but before the dice have been coupled to thepackage substrate.

FIG. 5A shows a top view of an embodiment of the invention after thedice have been coupled to the package substrate.

FIG. 5B shows a side view of an embodiment of the invention after thedice have been coupled to the package substrate.

FIG. 6 shows a top view of an embodiment of the invention in a sequenceof molding machines.

FIG. 7 shows a side view of an embodiment of the invention in a moldchase.

FIG. 8 shows a side view of an embodiment of the invention in a moldchase during the molding process.

FIG. 9 shows a side view of a cross section of an embodiment of theinvention after removal of the mold chase.

FIG. 10 shows an embodiment of the invention in the form of a singulateddie package.

FIG. 11 shows an embodiment of the invention in the form of a flowchartfor using flow modifiers.

FIG. 12 shows an embodiment of the invention in the form of a flowchartof instructions provided by a machine-readable medium.

DETAILED DESCRIPTION OF THE INVENTION

In the following description numerous specific details are set forth.However, it is understood that the embodiments of the invention may bepracticed without these specific details. In other instances, well-knowndetails such as particular materials or methods have not been shown indetail in order not to obscure the understanding of this description.

Referring to FIG. 1, a side view of an embodiment of the invention isshown having two separate mold flows. In one embodiment of theinvention, the top mold flow 121 flowing between the top of the dice103, 105, and 107 and the mold chase 120, and the bottom mold flow 123between the dice 103, 105, and 107 and the package substrate 101, arekept separate by flow modifier 109. By splitting the mold flow into twoseparate mold flows using the flow modifier 109, the top mold flow 121may be prevented from wrapping over the dice 103, 105, and 107. Withoutthe flow modifier 109, the bottom mold flow 123 and the resultingoverwrapping mold flow (not shown) from the top of the dice 103, 105,and 107, may trap air pockets (not shown) under the dice 103, 105, and107.

In one embodiment package substrate 101 is a printed circuit board (PCB)and dice 103, 105, and 107 are flip chips. Other embodiments may useother types of package substrates and dice. Electrical connections, suchas but not limited to C4 solder bump 111, may be used to couple the dice103, 105, and 107 to the package substrate 101 before or after the flowmodifier 109 is applied. Electrical connections may be applied toconnect electrical circuits in the package substrate 101 to electricalcircuits in dice 103, 105, and 107. The flow modifier 109 may be asolder resist mask. In some embodiments the solder resist mask materialcomprises thermoset resins for eg epoxy, polyimide and polyacrylate.Other embodiments may use other types of material in flow modifier 109.In one embodiment, the composition of the solder bump 111 may becomprised of lead and tin. In other embodiments, other solder bumpcompositions may be used.

Referring to FIG. 2, a top view of an embodiment of the invention isshown with device placement locations. Referring also to FIG. 1, becausethe flow modifier 109 may be applied before the dice 103, 105, and 107are coupled to package substrate 101, the device placement locations203, 205, and 207 for the dice 103, 105, and 107, respectively, may beprearranged in a pattern on a package substrate 101. The surface of thepackage substrate 101 between each device placement location 203, 205,and 207 may be covered with flow modifier 109 to a height sufficient toseparate the mold flow into two flows: a top mold flow 121 and a bottommold flow 123. Note: Although specific devices and locations areidentified in the figures and referred to in the text, the identifiedlocations and devices may be considered generic examples of similarlocations and devices that are also shown in the figures but may beunlabeled.

For the height of the flow modifier 109 to be sufficient to separate themold flow into two flows, the flow modifier 109 may need to be at leastequal to the distance between the surface of the package substrate andthe surface of the die that is nearest the package substrate. In oneembodiment the flow modifier height is approximately equal to thediameter of a solder bump 111. In another embodiment the flow modifierheight is equal to a diameter of a solder bump 111 plus a thickness(referring to the smallest thickness) of a die 103. Other heights mayalso be used. In addition, the surface of the package substrate 101between the outer sides of the device placement locations 203, 205, and207 and the outer edge of the package substrate 101 may also have flowmodifier 109 applied to keep the top mold flow 121 and the bottom moldflow 123 separate.

Referring to FIG. 3, a side view of an embodiment of the invention isshown with gap height indicators used for determining the height of theflow modifiers. The top gap height 303 is dependent on the distancebetween the top of the dice 103, 105, and 107 and the mold chase 120(seen in FIG. 1). The height of the flow modifier on the packagesubstrate 101 may depend partially on the standoff height 305. Theheight of the flow modifier may need to be at least as high as thestandoff height 305 to prevent the top mold flow 121 (seen in FIG. 1)from getting under dice 103, 105, and 107. In one embodiment, the flowmodifier may extend from an upper surface 309 of the package substrate101 to a height at least even with a lower surface 307 of a die, such asdie 103. In one embodiment of the invention, the standoff height 305between the dice 103, 105, and 107 and the package substrate 101 may bedependent on the height of solder bumps 111. Because the flow modifier109 may be applied before or after the dice 103, 105, and 107 are laiddown, the height and placement of the flow modifier 109 may bepredetermined. The placement of the flow modifier 109 may bepredetermined by arranging the device placement locations 203, 205, and207 (seen in FIG. 2) in order to determine where the flow modifier 109may be applied.

Referring to FIG. 1 and FIG. 3, top gap height 303 may vary dependingupon the thickness of dice 103, 105, and 107 and the height of theelectrical connections used, such as but not limited to solder bumps111. The top mold flow 121 may have a height equal to top gap height 303and may have a flow speed governed by the equation:

$v = \frac{\Delta\; p\; d^{2}}{32\mspace{11mu}\mu\; l}$where Δp is the pressure drop applied to move the molding compound, μ isthe melt viscosity of the molding compound, l is the cavity length, andd is the gap the molding compound is flowing through. Because thestandoff height 305 may be small compared to the top gap height 303, thebottom mold flow 123 may progress much slower than the top mold flow121. In another embodiment of the invention, the top gap height 303 maybe smaller than the standoff height 305 which may cause the bottom moldflow 123 to have a higher flow speed than the top flow front 121.However, if flow modifier 109 is applied to the package substrate 101 toa height at least equal to the standoff height 305, the top mold flow121 and the bottom mold flow 123 may be kept substantially separate.

Heights of the flow modifier 109 may depend on the height of the solderbump 111. For example, the solder bumps 111 may have a height betweenabout 25 microns and about 100 microns, and the minimum height of theflow modifier 109 may similarly be between about 25 microns and about100 microns. Other heights of the flow modifier 109 may also be withinthe scope of an embodiment of the invention, e.g., the height of theflow modifier 109 may be between about 75 microns and about 400 microns.Other heights of the solder bumps 111 and flow modifier 109 are alsopossible.

Referring to FIG. 4A an embodiment of the invention is shown with flowmodifier applied to the package substrate 101 before the coupling of thedice 103, 105, and 107 (seen in FIG. 1). After the device placementlocations 203, 205, and 207 have been determined, the flow modifier 109may be applied using various techniques, e.g., a screen printingprocess. The stencil used in the screen printing process may be designedusing the device placement locations 203, 205, 207, 209, and 211. Othermethods of applying the flow modifier 109 may also be within the scopeof the invention.

Referring to FIG. 4B, an embodiment of the invention is shown in theform of a side view of a package substrate 101 with flow modifier 109.The view in FIG. 4B is the same as the view in FIG. 1. The height of theflow modifier 109 may be predetermined based on a distance from thesurface of a die, such as die 103 (seen in FIG. 1), used to couple thedie 103 to the package substrate 101, to the surface of the packagesubstrate 101 that is coupled to the die 103. The flow modifer 109 maythen be applied to this predetermined height before the dice 103, 105,and 107 (seen in FIG. 1) are coupled to the underlying substrate 101.

Referring to FIG. 5A, a top view of an embodiment of the invention isshown after coupling the dice to the package substrate in the deviceplacement locations. Referring to FIG. 5B, a side view of an embodimentof the invention is shown after coupling the dice to the packagesubstrate in the device placement locations. The view in FIG. 5B is atright angles to the view in FIGS. 1 and 4B. In one embodiment of theinvention, the dice 103, 105, and 107 are coupled to the packagesubstrate 101 after the flow modifier 109 has been applied. In anotherembodiment of the invention, the flow modifier 109 may be applied afterthe dice 103, 105, and 107 have been coupled to the package substrate101. The dice 103, 105, and 107 may be coupled to the package substrate101 by a reflow process with solder to form solder bumps 111. Othermethods of coupling the dice 103, 105, and 107 may also be within thescope of the invention. In one embodiment some dice (e.g. dice 521 and523) may be coupled to the package substrate before the flow modifier109, and other dice (e.g., dice 103, 105, 107) may be coupled to thepackage substrate after the flow modifier. Other sequences of couplingthe dice 103, 105, 107, 521, and 523 may also be within the scope of theinvention.

Referring to FIG. 6, a top view of an embodiment of the invention isshown with the package substrate and coupled dice placed into a moldingmachine. For purposes of illustration, the mold chases 120 (seen in FIG.7) have been made transparent. After coupling the dice 103, 105, and 107to the package substrate 101, and after the flow modifier 109 has beenapplied, the package substrate 101 and coupled dice 103, 105, and 107may be placed into a molding machine with mold runners 601. Moldingcompound may be pushed through mold runners 601. Mold runners 601 mayhave various shapes, configurations, and thicknesses, as long as theyare able to deliver the mold compound to the assembled package substrateand dice.

Referring to FIG. 7, a side view embodiment of the invention is shownwith the package substrate 101, dice 103, 105, and 107, and flowmodifier 109 placed in a molding machine. Top mold flow 121 (seen inFIG. 1) may flow between the top mold chase 120 in space 119 on top ofdice 103, 105, and 107. The bottom mold flow 123 (seen in FIG. 1) mayflow between the die 103, 105, 107 and the package substrate 101.

Referring to FIG. 8, in some embodiments of the invention, the moldingprocess occurs with a molding compound under pressure and at a hightemperature. The molding compound may flow between package substrate 101and mold chase 120 by flowing over, under, and around the dice 103, 105,and 107 in a laminar flow governed by the Hagen-Poissule equation:

${\Delta\; p} = \frac{32\mspace{11mu}\mu\; v\; l}{d^{2}}$Applying the equation to the dice 103, 105, and 107 and the packagesubstrate 101, Δp is the pressure drop applied to move the moldingcompound, μ is the melt viscosity of the molding compound, ν is the flowspeed of the flow front, l is the cavity length, and d is the height ofthe gap the molding compound is flowing through.

As the molding compound is applied, part of the molding compound mayflow over the dice 103, 105, and 107, such as top mold flow 121, whilepart of the molding compound flows under the dice 103, 105, and 107 inthe gaps under the dice 103, 105, and 107 created by the solder bumps111. Without flow modifier 109 the molding compound may flow around andover the dice 103, 105, and 107 and trap air under the dice 103, 105,and 107.

Keeping the top mold flow 121 separate from the bottom mold flow 123 mayprevent the molding compound flow fronts from wrapping around a die'sedge and trapping air bubbles under the dice. Substrates, such as butnot limited to dice 103, 105, and 107, each with a top (first) andbottom (second) surface, may be coupled to an package substrate 101,also with a top (third) and bottom (fourth) surface, by electricalconnections, such as but not limited to solder bump 111. Solder bump 111may couple the top surface 110 of a first substrate, such as but notlimited to an package substrate 101, to a bottom surface 112 of a secondsubstrate, such as but not limited to die 103.

The bottom mold flow 123 may be separated from the top mold flow 121 bya material boundary such as but not limited to flow modifier 109. Othermaterial boundaries may also be within the scope of an embodiment of theinvention. The flow modifier 109 may prevent the top mold flow 121 fromwrapping over the sides of or under dice 105 and 107. Because the topmold flow 121 may not flow into the bottom mold flow 123, the bottommold flow 123 may push all the way through the gaps under the dice 103,105, and 107 without trapping air under the dice 103, 105, and 107. Thebottom mold flow 123 may then be between the first substrate, such asbut not limited to the package substrate 101, and the second substrate,such as but not limited to die 103, at the same time as the top moldflow 121 is flowing over die 103 without trapping air under the die 103.

Referring to FIG. 9, a cross section of the embodiment of FIG. 8 isshown after the mold chase 120 has been removed, with cured top moldcompound 901 and cured bottom mold compound 905. After the top mold andbottom mold compounds have been applied and have solidified, a moldedmatrix array package 921, comprising a package substrate 101, flowmodifier 109, substrates 103, 105, and 107, top mold compound 901 andbottom mold compound 905, may be removed from the mold chase 120 as anintegrated unit.

Referring to FIG. 10, an embodiment of the invention is shown after eachdie has been singulated. Die 103 may be singulated along its sides fromthe rest of the dice 105 and 107 (not shown).

The flow modifier 109 may remain with the singulated package 103 or maybe removed. The die 103 may be singulated from the rest of the dice 105and 107 using a singulating saw. Other singulating methods may also beused.

Referring to FIG. 11, an embodiment of the invention is shown in theform of a flowchart for using flow modifiers. At block 1101 a substrate,such as but not limited to a package substrate, may be provided. Atblock 1103, a device placement location on the substrate may bedetermined for a die to be coupled to the substrate. At block 1105, aflow modifier height may be determined. In one embodiment the height isat least equal to a distance from a top surface of the substrate to abottom surface of the die when the die is coupled to the substrate. Atblock 1107, a flow modifier may be coupled to the substrate adjacent tothe device placement location, with the height of the flow modifierextending approximately to the height determined in block 1105. In oneembodiment a single flow modifier may be coupled to the surface of thesubstrate adjacent to multiple device locations. At block 1109, a diemay be coupled to the substrate at the device placement location. Invarious embodiments, the die may be coupled to the substrate before,after, or simultaneously with the flow modifier. At block 1111, amolding compound may be applied to the substrate/dice assembly. In oneembodiment a single application of molding compound is diverted to twoseparate flows by the flow modifier, with one flow going over the dieand the second flow going between the die and the substrate. In anotherembodiment two separate flows of molding compound are applied to thesubstrate/die assembly, one flow being directed above the die and theother flow being directed between the die and the substrate. In oneembodiment, low pressure is applied at block 1113 to help the moldingcompound to move more easily.

Referring to FIG. 12, an embodiment of the invention is shown in theform of a flowchart of instructions provided by a machine-readablemedium to one or more processors that control one or more devices. Amachine-readable medium includes any mechanism that provides (i.e.,stores and/or transmits) information in a form readable by a machine(e.g., a computer). For example, a machine-readable medium may includeread only memory (ROM); random access memory (RAM); magnetic diskstorage media; optical storage media; flash memory devices; electrical,optical, acoustical or other form of propagated signals (e.g., carrierwaves, infrared signals, digital signals, etc.); etc. At block 1201, adevice placement location may be determined for placement of a die on asubstrate. At block 1203, a flow modifier height may be determined, withthe height to be at least equal to a distance from a top surface of thesubstrate to a bottom surface of the die when the die is coupled to thesubstrate. At block 1205, a flow modifier may be coupled to thesubstrate adjacent to the device placement location, and extending to aheight substantially equal to the flow modifier height determined inblock 1203. At block 1207, a die may be coupled to the substrate at thedevice placement location determined in block 1201. In variousembodiments, the die may be coupled to the substrate before, after, orat the same time as the flow modifier is coupled to the substrate. Atblock 1209, a first molding compound may be applied over the die. Atblock 1211, a second molding compound may be applied between the die andthe substrate. The first molding compound may be applied before, after,or simultaneously with the second molding compound, and may be the samemolding compound.

In one embodiment of the invention, flow modifiers may be used around adie coupled to a motherboard before the molding process. Anotherembodiment of the invention may be used in a direct chip process. Otherembodiments of the invention may involve any molded package using a flowmodifier to split a mold flow into two or more mold flows to preventtrapped air. In addition, other processes may be used in addition toapplying flow modifiers to prevent trapped air. For example, in oneembodiment of the invention, a low pressure may be pulled during themolding process over the dice to reduce trapped air during the moldingprocess. In another embodiment of the invention, holes may be putthrough the package substrate below the dice 103, 105, and 107 (seen inFIG. 1) to allow air to escape as the molding compound flows over andunder the package. In one embodiment of the invention, both low pressureand air holes may be used in conjunction with flow modifiers to reducetrapped air.

Although the previous figures depict flow modifier 109 being placedalong the full length of two opposite sides of each die, otherconfigurations may also be used. For example, flow modifier 109 may beplaced along one side or three sides of a die. Also, flow modifier 109may be placed along only a portion of any given side of the die. Thehorizontal gap between the flow modifier and the side of the die mayhave various dimensions. In one embodiment, this gap is effectivelyzero, to prevent air from escaping through the gap. In anotherembodiment, this horizontal gap may be sufficiently large for air toescape, but small enough to effectively prevent the flow of the moreviscous molding material from passing therethrough. The preferred gapsize may depend on various factors, such as the temperature of themolding material, the viscosity of the molding material at thattemperature, the size of any solid filler materials in the moldingmaterial, etc.

While the invention has been described in terms of several embodiments,those of ordinary skill in the art will recognize that the invention isnot limited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. The description is thus to be regarded as illustrative insteadof limiting.

1. A method comprising: coupling a pair of flow modifiers to a firstsubstrate to form an adjacent border extending along only two sides of adevice placement location; coupling a second substrate to said firstsubstrate at said device placement location, leaving a gap between thecoupled surfaces having a height less than or equal to that of said pairof flow modifiers; applying a first molding compound over said secondsubstrate; and applying a second molding compound between said firstsubstrate and said second substrate, wherein said pair of flow modifierssubstantially separates a flow of said first molding compound from aflow of said second molding compound.
 2. The method of claim 1 whereinsaid height of said flow modifiers is a distance approximately between75 microns and 400 microns.
 3. The method of claim 1 further comprisingapplying a low pressure over said substrates.
 4. The method of claim 1wherein said applying the first molding compound over said secondsubstrate and applying said second molding compound between said firstsubstrate and said second substrate happen at substantially the sametime.
 5. The method of claim 1 wherein applying said first moldingcompound over said second substrate happens before applying said secondmolding compound between said first substrate and said second substrate.6. The method of claim 1 wherein applying said first molding compoundover said second substrate happens after applying said second moldingcompound between said first substrate and said second substrate.
 7. Themethod of claim 1 wherein the second substrate is coupled to said firstsubstrate after said pair of flow modifiers are deposited.
 8. The methodof claim 1 wherein each of said pair of flow modifiers further abut anadjacent device placement location.
 9. The method of claim 1, whereinsaid pair of flow modifiers have a height greater than the gap betweenthe coupled surfaces.
 10. The method of claim 1, wherein said firstmolding compound has the same composition as said second moldingcompound.
 11. The method of claim 1, wherein said second substrate is anintegrated circuit die and said first substrate is a package substrate.12. A method comprising: coupling a pair of flow modifiers to a firstsubstrate to form an adjacent border extending along only two sides of adevice placement location; coupling a second substrate to said firstsubstrate at said device placement location, leaving a gap between thecoupled surfaces having a height less than or equal to that of said pairof flow modifiers; introducing a first molding compound over said secondsubstrate and a second molding compound between said first substrate andsaid second substrate and; bifurcating each of said pair of flowmodifiers to singulate said device placement location from an adjacentdevice placement location.
 13. The method of claim 12 wherein said pairof flow modifiers have a height greater than the gap between the coupledsurfaces.
 14. The method of claim 12 wherein said first molding compoundhas the same composition as said second molding compound.
 15. A methodcomprising: providing a substrate having a first and second dieplacement location; forming a on the substrate a pair of flow modifiersbordering two opposite sides of said first and second die placementlocation; coupling a first die to said substrate at said first dieplacement location and coupling a second die to said substrate at saidsecond die placement location, wherein said coupling forms a gap betweenthe coupled surfaces having a height less than or equal to that of saidfirst and second flow modifier; introducing molding compound to a sideof said first die to overmold said first and second die; and introducingmolding compound to a side of said first die to underfill said gapbetween said coupled surfaces of said first and second die to saidsubstate, wherein said pair of flow modifiers substantially separates anovermold flow from an underfill flow.
 16. The method of claim 15,wherein said overmold and said underfill are initiated at substantiallythe same time.
 17. The method of claim 15, wherein said overmold isperformed before said underfill.
 18. The method of claim 15, whereinsaid overmold is performed with the same molding compound as saidunderfill.
 19. The method of claim 15, further comprising singulatingsaid first and second die from adjacent die placement locations bybifurcating each of said pair of flow modifiers.